Devices for separating a copy sheet from a moving photoconductive imaging surface of an electrophotographic copier following transfer of a developed image to the sheet, variously referred to as pickoffs or strippers, are well known in the art. Thus, one type of pickoff known in the art uses a mechanical element such as a blade which rides on the imaging surface to intercept the leading edge of a copy sheet as it moves past the element. One of the advantages of a pickoff of this type is that it allows intimate contact between the entire width of the copy sheet and the imaging surface in the transfer station. However, if such a pickoff is allowed to ride continuously on the photoconductive imaging surface, it will eventually abrade the photoconductive surface, thus reducing its useful life.
Moreover, if a contacting pickoff blade is maintained at the same position along the width of the photoconductor, and is electrically grounded to prevent charge buildup, it will adversely affect the photoconductor electrically along the line of contact resulting in light gray voids on copies. This effect is believed to occur because the blade discharges the contacted portion of the photoconductor, bringing its electrical potential well below that of adjacent surface portions, which retain a residual of their original charge. This charge differential remains to a substantial degree even when the surface is subsequently charged in preparation for another copy cycle. As a result, the visible image along the line of contact with the pickoff blade, which is determined by the electrical charge, is noticeably less black than the image on adjacent areas.
It is known in the art to move a contacting mechanical pickoff element transversely of the imaging surface so as to minimize the wear of any one longitudinal surface portion contacted by the element. Thus, Kono et al U.S. Pat. No. 4,252,310 and Golz et al U.S. Pat. No. 4,168,902 discloses various systems for shifting a pickoff element transversely of the direction of movement of a photoconductor to alter the point of engagement. However, since these systems only move the pickoff over a small portion of the width of the photoconductor, the wear along the contacted portion remains relatively high. In addition, the systems for imparting a translatory motion to the pickoff element are relatively complicated.
It is also known in the art, as shown in the Kono et al patent referred to above, to lift the pickoff element from the photoconductor during certain portions of the copy cycle to minimize the total contact time. However, if pickoff retraction is timed only with regard to the passage of the sheet, there remains the possibility that the pickoff will accumulate heavy toner deposits, especially in a liquid-developer copier having a development electrode which is periodically cleaned by electrically attracting toner material from the electrode onto a non-image portion of the photoconductor.